1. Field of the Invention
The present invention relates to an array antenna reception apparatus installed in a base station for removing another user interference under antenna directivity control and, more particularly, to an array antenna having antenna elements linearly laid out on each side of a polygon.
2. Description of the Prior Art
In a cellular mobile communication system and the like, the following method is examined. A directional pattern which maximizes the reception gain in a desired signal arrival direction is formed using an adaptive antenna made up of a plurality of antenna elements, and interference from another user and interference by a delayed wave are removed in reception. As a radio transmission method expected for a large subscriber capacity, the CDMA method receives a great deal of attention.
FIG. 1 is a block diagram showing an example of a conventional array antenna reception apparatus using the CDMA method.
The conventional array antenna reception apparatus is constituted by an antenna 20 having a plurality of antenna elements 211 to 21M laid out circularly, one adaptive receiver 22, and a determination circuit 5.
The antenna 20 is made up of the M antenna elements 211 to 21M laid out circularly. Each of the antenna elements 211 to 21M is not particularly limited in horizontal plane directivity and may take omnidirectivity or dipole directivity. The M antenna elements 211 to 21M are close to each other so as to establish correlations between antenna reception signals, and receive signals obtained by code-multiplexing a desired signal and a plurality of interference signals. In the following processing, since signals are digitally processed in the baseband, M antenna reception signals S1 to SM are frequency-converted from the radio band to the baseband and A/D-converted.
The determination circuit 5 receives a demodulated signal for a user as an output from the adaptive receiver 22 and performs hard determination for the demodulated signal, thereby outputting a user determination symbol. Here, it should be noted that only one of the determination circuit 5 is shown in FIG. 1, but other circuits are omitted.
FIG. 2 is a block diagram showing the adaptive receiver 22 in the conventional array antenna reception apparatus.
The adaptive receiver 22 is constituted by despread circuits 61 to 6M, weighting synthesizer 7, demodulator 10, complex multiplier 13, subtracter 14, delay circuit 15, and antenna weight control circuit 16. The adaptive receiver 22 receives the antenna reception signals S1 to SM received by the M antenna elements 211 to 21M laid out circularly, and the user determination symbol as an output from the determination circuit 5, and outputs a demodulated signal for a user.
The despread circuits 61 to 6M calculate correlations between the antenna reception signals S1 to SM and a user spread code C. Assuming that the spread code C is a complex code made up of two quadrature codes CI and CQ, the despread circuits 61 to 6M can be realized by one complex multiplier and averaging circuits over the symbol section. The despread circuits 61 to 6M can also be realized by a transversal filter arrangement with a tap weight C.
The weighting synthesizer 7 comprises complex multipliers 81 to 8M and adder 9. The weighting synthesizer 7 multiplies outputs from the despread circuits 61 to 6M by antenna weights Wr1 to WrM, and adds them to generate a signal received with a directional pattern unique to a desired signal.
The demodulator 10 comprises a transmission path estimation circuit 11 and complex multiplier 12. The product of an output from the weighting synthesizer 7 and the complex conjugate of a transmission path estimation output is the demodulated signal for a user as an output from the adaptive receiver 22.
The complex multiplier 13 multiplies the user determination symbol by the transmission path estimation output. In multiplying the user determination symbol by the transmission path estimation output, only a component about the phase of the estimation value can be multiplied, and an amplitude obtained by another means can be multiplied. This another means is one for obtaining the amplitude by measuring reception power or the like.
The subtracter 14 calculates the difference between an output from the complex multiplier 13 and an output from the weighting synthesizer 7, and detects an antenna weight control error e.
The delay circuit 15 delays outputs from the despread circuits 61 to 6M in accordance with the processing times of the weighting synthesizer 7, demodulator 10, subtracter 14, and the like.
The antenna weight control circuit 16 calculates the antenna weights Wr1 to WrM from the antenna weight control error e and outputs from the delay circuit 15. The antenna weight control circuit 16 adaptively controls the antenna weights Wr1 to WrM based on the MMSE standard so as to minimize the mean square value of the antenna weight control error e. When the LMS algorithm is employed as an update algorithm with a small arithmetic amount, the antenna weights Wr1 to WrM are given by
Wr(i+1)=Wr(i)+xcexcr(ixe2x88x92Ddem)e*(i)xe2x80x83xe2x80x83(1)
where Wr(i) (column vector having M elements) is the antenna weight of the ith symbol, r(i) (column vector having M elements) is the antenna reception signal, xcexc is the step size, Ddem is a delay time given by the delay circuit 15, and * is the complex conjugate. From equation (1), the antenna weights Wrl to WrM are updated every symbol. The adaptive control convergence step may use a known symbol instead of the determination symbol.
The M antenna reception signals S1 to SM contain desired (user) signal components, interference signal components, and thermal noise. Each of the desired signal component and interference signal component contains a multipath component. In general, these signal components arrive from different directions. In forming a reception directional pattern, the conventional array antenna reception apparatus shown in FIG. 1 uses an antenna having antenna elements laid out circularly. Thus, a directional pattern with almost uniform reception gains in all the signal arrival directions can be formed.
However, first, the conventional array antenna reception apparatus shown in FIG. 1 cannot attain a high reception gain proportional to the number of antenna elements.
This is because the directional pattern with almost uniform reception gains in all the signal arrival directions is formed by circularly laying out antenna elements, and the reception gain cannot be optimized.
Second, as the number of antenna elements increases, the conventional array antenna reception apparatus shown in FIGS. 1 and 2 decreases in adaptive convergence and stability in forming a directional pattern in the desired user direction.
This is because in the antenna having antenna elements laid out circularly, all the antenna elements must be simultaneously adaptively controlled.
The present invention has been made in consideration of the above situation in the prior art, and has as its object to provide an array antenna reception apparatus which can attain a high reception gain proportional to the number of antenna elements and is excellent in adaptive control convergence and stability in forming a directional pattern in the user direction.
To achieve the above object, an array antenna reception apparatus according to the main aspect of the present invention is constituted as follows. Antenna elements are linearly laid out on each side (sector) of a polygon, a directional pattern for suppressing interference with another user or multipath is independently formed for each sector, and weighting synthesis is done between sectors. More specifically, the array antenna reception apparatus comprises an array antenna having M (M is an integer of not less than 1) antenna elements linearly laid out on each side (sector) of a polygon having K (K is an integer of not less than 3) sides, K adaptive receivers each for receiving reception signals from the M antenna elements for a corresponding sector, independently forming a directional pattern having a gain in a desired signal direction for the sector, receiving a desired signal, and suppressing an interference signal, and a demodulated signal synthesizer for receiving K demodulated signals as outputs from the K adaptive receivers, weighting and synthesizing the signals, and outputting a demodulated signal for a user.
In the present invention, since the antenna elements are linearly laid out every sector, a directional pattern with a high reception gain substantially proportional to the number of antenna elements can be formed in a direction perpendicular to each straight line (each sector side). Since the directional pattern is independently formed for each sector, the number of antenna elements simultaneously adaptively controlled can be decreased. Even if the number of antenna elements increases, the adaptive convergence and stability are kept high in forming a directional pattern in a desired user direction.